Tuning coil with movable magnetic core



Jan. 20,1970 TAMAKI oH sm EITAL 3,491,320

TUNING COIL WITH MOVABLE MAGNETIC CORE Criginal Filed March 29, 1967 3Sheets-Sheet l F/g. PR/OR ART 2 PR/OR ART l6 f (X'lOOK /s) |4 1'0 2'0 50 L (mm) INVENTORS TAMAKI OHASHI TAKESHI ISHlNO IsAo YOKOYAMA ATTY6.

Jan. 20,1976 TAMAKI OHASHI ETAL 3,491,320

TUNING COIL WITH MOVABLE MAGNETIC CORE Original Filed March 29, 1967 3Sheets-Sheet 2 R 3 PR/OR ART 6 v 4- IO (0 2'0 30 L (mm) 4 5 PRIOR ARTPRIOR ART 2 Mill/fir INVENTORS TAMAKI OHASH TAKESHI ISl-HNO IsAoYOKOVAMA 5J3 W0%) W7 ATTYS,

Jam. 20,1970 TAMAKI oHAsm L 3,491,320

TUNING con; wxwa MOVABLE mimic CORE Original Filed March 29, 1967 aSheets-Sheet s INVENTORS TAMAK/ OHASHI TAKESHI ISHINO I SAO YOKOYAMAArrvs.

United States Patent TUNING COIL WITH MOVABLE MAGNETIC CORE TamakiOhashi, Tokyo, and Takeshi lshino and Isao Yokoyama, Akita-ken, Japan,assignors to TDK Electronics Co., Ltd., Tokyo, Japan, a corporation ofJapan Continuation of application Ser. No. 626,742, Mar. 29,

1967. This application Jan. 31, 1969, Ser. No. 798,254 Int. Cl. H01f21/10, 21/06 US. Cl. 336-87 1 Claim ABSTRACT OF THE DISCLOSURE Avariable inductance coil including a solenoid type coil, an adjustingcore made of magnetic material and mounted for axial movement relativeto the coil, and a fixed magnetic sleeve disposed around the coil toenclose the same. The fixed magnetic sleeve extends longitudinallybeyond the coil at the end where the adjusting core is located so thatthe fluctuation of the electric characteristics of the adjusting core isminimized.

This application is a continuation of application Ser. No. 626,742,filed Mar. 29, 1967, now abandoned.

This invention relates to a variable inductance (,u tuning) coil to beused in a method of a tuning system, for example, in a car radio set.

A dust core has been used exclusively for a conventional variableinductance coil. As the advantages of such a dust core, it can beenumerated that the temperature coefiicient is small, and thatapplicable frequency band is wide. On the other hand, its permeabilityis so low that the long stroke, as the variable inductance, is essentialto cover a certain tuning frequency range, the gap between the sleevecore and adjusting core must be small and the close tolerance of thecores must be required.

Instead of the above described dust cores, there have been used ferritecore in Japan since some time ago to eliminate the defects of such dustcores. However, inherently ferrite is a material so high in thepermeability that there have been many faults due to unknown factors andhave been much troublesome especially in the tracking process of theradio set production.

As illustrated in FIG. 1, in a conventional ferrite variable inductancecoil, a coil 2 is wound on a bobbin 1, a fixed magnetic sleeve core 3(which shall be known as a sleeve core hereinafter) shorter than thecoil 2 is applied outside the said coil and a shield case 4 is furtherapplied outside the said sleeve core so that an adjusting core (movingcore) 5 for tuning may be moved in the directions indicated by thearrows x and x for tuning.

Usually, the tuner for the car radio set has an antenna coil, tuningcoil and oscillating coil or further a high frequency amplifying c-oil.As in adjusting them the permeability of the adjusting core isfluctuated, the position of the adjusting core is also fluctuated incase of obtaining the same inductance. Consequently, in mass-producedsets incorporating such a kind of cores, the frequency f-stroke L curvesas shown, for example, in FIG. 2 of these tuners do not agree with eachother. Besides, the curve of each coil in one tuner does not becomeparallel. Therefore, there is a great defect that, in case they areincorporated into radios, tracking will become diflicult. For example,in case, in FIG. 1, the core 5 is of a diameter of '5 mm. and a lengthof 30 mm. and the sleeve core 3 is of an outside diameter of 15 mm., aninside diameter of 12 mm. and a length of 14 mm, the average tuningfrequency by the stroke L of the tuning core 5 and five tuning coreswill be :as shown in FIG. 2. FIG. 3 shows the fluctuation of the tuningfrequency by a range R. In FIG. 3, the curve 10 represents thefluctuation of the tuning frequency of the same as is shown in FIG. 2and the curve 11 represents the case according to the present invention.As evident from FIG. 2 and the curve 10 in FIG. 3, even if the dimensionallowance is $0.05 mm., the fluctuation between respective samples willbe of a considerably large value as shown by the curve 10. Thus trackinghas been diflicult.

As a countermeasure against it, as shown in FIG. 4, notches 6 and 6'have been made in a part of the adjusting core 5 or, as shown in FIG- 5,the winding pitch of the coil 2 has been varied. However, in eithermanner, the operation has been complicated, the production cost has beenhigh and yet no sufficient effect has been obtained.

In the present invention, in order to reduce such fluctuation, theinteraction between the sleeve core and the adjusting core have beeninvestigated on the permeability of the cores and the relativepositioning. In the present invention, as shown in FIG. 6, the sleevecore 3 is made longer by k than the coil 2 and FIG. 7 shows thefluctuationR-stroke L as parameter k, which is shown in FIG. 6.

In FIG. 7, the curves A, B and C represent fluctuations in case dustcores having a value of ,u =20 were used for the sleeve core andadjusting core. It is found that, when the value of k of the sleeve corewas varied to 0 mm. (represented by the curve A), 5 mm. (represented bythe curve B) and 10 mm. (represented by the curve C), the reduction ofthe fluctuation was shown as the value of k was varied toward k: 10 mm.,and the deviation was 4 kc./ sec. in the value of k=10 mm. On the otherhand, when only the sleeve core was made from the material of :450, thefluctuation reduced gradually as in 0, 2, 4, 6, 8 and 10 mm. in FIG. 7,and was of a practically satisfactory value at k: 10 mm.

From the above, if ,u, of the material of the sleeve core is made aslarge as possible, m of the adjusting core is made small and the lengthof the sleeve core is made longer than the coil, the fluctuation oftuning frequency will be minimized. Therefore, in the variableinductance coil represented by the curve 10 in FIG. 3, in case theadjusting core was made from dust core and the sleeve core (of :50) wasmade from ferrite 3 mm. longer than the coil, the fluctuation R was asin the curve 11 in FIG. 3.

As described above, according to the present invention, by means ofmaking the permeability ,u, of the material of the sleeve core more than50 and making the sleeve core slightly longer than the coil, thefluctuation of the inductance of the variable inductance coil can bemade small and tracking can be made easy.

While there has been described in connection with the preferredembodiment of this invention, it will be obvious to those skilled in theart that various changes and modifications may be made therein withoutdeparting from the invention, and it is aimed, therefore, to cover inthe appended claim all such changes and modifications as fall within thetrue spirit and scope of the invention.

We claim as our invention.

1. A variable inductance coil comprising a single solenoid type coilwound on a fixed hollow bobbin, said single coil being wound withuniform spacing over its entire length, an adjustable core made ofmagnetic material and mounted for axial movement within one end of saidhollow bobbin relative to said coil, a. fixed magnetic sleeve disposedaround said coil and made of ferrite having a permeability of more than50 and greater than the permeability of said core, said sleeve extendingbeyond said coil by a distance sufficientto minimize fluctuations in thetuning frequency of said coil, and a shield casing disposed around saidfixed magnetic sleeve.

References Cited UNITED STATES PATENTS Weis 336134 Harvey 33687 Berg336132 XR OTHER REFERENCES Bozorth: Ferromagnetism, D. Van NostrandCompany Inc., March 1951, pp. 246 and 870-871.

THOMAS J. KOZMA, Primary Examiner US. Cl. X.R.

Conradet a1 336136XR 10 336432 136

